Efforts to reduce sulfur emissions in the gaseous products from combustion of a sulfur-containing fuel have been made in varying directions. Some processes attempt to reduce or eliminate the sulfur in the fuel prior to its combustion. Other processes propose the addition of compounds to the combustion zone which will in some manner change the nature of the sulfur compounds such that they may be more readily removed from the combustion products. And yet other processes remove sulfur compounds from the gaseous combustion products by chemical reaction.
U.S. Pat. No. 4,185,080, issued Jan. 22, 1980 to Rechmeier discloses a combustion gas desulfurization process wherein a powdered sorbent such as limestone or dolomite is added to a combustion zone, and a portion of the solids collected from the flue gas is reactivated and can be returned for injection into or downstream of the combustion zone. An earlier U.S. Pat. No. 3,320,906 to Domahidy, issued May 23, 1967, teaches mixing limestone with the coal being fed to a boiler, with the flue gas then being passed to a wet scrubber for additional capture of sulfur compounds by the calcined limestone.
The approach of chemical treatment to effect flue gas desulfurization can be further subdivided into wet scrubbing wherein a solution or suspension of reagent both enters and leaves the flue gas contacting zone in liquid state, spray drying wherein a solution or suspension of reagent enters the flue gas contacting zone in liquid state but is dried to produce a powdered solid leaving the contacting zone, and dry treatment wherein the treating reagent is a solid state powder both entering and leaving the contacting zone.
Illustrative of the wet scrubber approach is U.S. Pat. No. 3,928,537, issued Dec. 23, 1975 to Saitoh et al, which discloses contacting the exhaust gas with an aqueous solution of an organic acid to form a soluble sulfite or sulfate. The sulfite or sulfate is removed, and the organic acid regenerated, by a second step comprising reaction with a calcium compound such as an inorganic or an organic acid salt, for example calcium hydroxide or calcium formate.
The spray dryer approach is illustrated for example by U.S. Pat. No. 4,279,873, issued July 21, 1981 to Felsvang et al, which discloses spraying a suspension of fresh slaked lime and recycled fly ash plus spent calcium compound into the hot flue gas in such a manner as to evaporate the slurry droplets to dryness; the resulting powdered solids are removed from the flue gas by a downstream electrostatic precipitator or bag filter.
U.S. Pat. No. 4,178,349, issued Dec. 11, 1979 to Wienert illustrates the dry treatment; it discloses mixing a dry, powdered lime-bearing material in a reactor, and subsequently separating the solids from the treated flue gas. Another patent, U.S. Pat. No. 4,442,079, issued Apr. 10, 1984, to Donelly et al, outlines a flue gas desulfurization process which is primarily adapted to the spray dryer procedure just discussed, but is also stated to be applicable to injection of dry sorbent at a point of relatively low flue gas temperature, with water being sprayed into the gas either upstream or downstream of the sorbent injection point.
Another "mixed" (wet and dry) flue gas desulfurization process is described in U.S. Pat. No. 4,388,281, issued June 14, 1983 to Holter et al. In this patent, dry sorbent can be mixed with the coal prior to combustion, but the main feature resides in splitting the flue gas into two parallel streams, with one stream being treated with fresh dry sorbent, and the other being wet scrubbed with sorbent solution containing e.g. piperazine.
Current thinking seems to be that no one of the above-discussed strategies is the unique answer to the sulfur emission problem, either for new installations or for retrofit on an existing installation. Rather, numerous site-specific factors such as proximity to reagent source, space availability, and extent of sulfur removal required, must enter into the selection at each plant.